In the gas turbine engine industry, there continues to be a need for improved corrosion- and oxidation-resistant protective coatings for nickel-base and cobalt-base superalloy components, such as blades and vanes, operating in the turbine section of the gas turbine engine. The use of stronger superalloys that often have lower hot corrosion resistance, the desire to use lower grade fuels, the demand for longer life components that will increase the time between overhaul and the higher operating temperatures that exist or are proposed for updated derivative or new gas turbine engines underscore this continued need.
Diffused aluminide coatings have been used to protect superalloy components in the turbine section of gas turbine engines. In a typical example, an aluminide coating is formed by electrophoretically applying an aluminum-base powder to a superalloy substrate and heating to diffuse the aluminum into the substrate. Such coatings may include chromium or manganese to increase the hot corrosion/oxidation resistance thereof.
To this end, it is known to improve the hot corrosion/oxidation resistance of simple diffused aluminide coatings by incorporating a noble metal, especially platinum, therein. Such platinum-enriched diffused aluminide coatings are now applied commercially to superalloy components by first electroplating a thin film of platinum onto a carefully cleaned superalloy substrate, applying an activated aluminum-bearing coating on the electroplated platinum coating and then heating the coated substrate at a temperature and for a time sufficient to form the platinum-enriched diffused aluminide coating on the superalloy substrate. Optionally, the platinum may be diffused into the substrate either prior to or after the application of the aluminum. The platinum forms an aluminide of PtAl.sub.2 and remains concentrated toward the outer surface regions of the coating.
Modified versions of the basic platinum-enriched diffused aluminide coating have been developed. One version on nickel-based alloys includes a two phase microstructure of NiAl(Pt) and PtAl.sub.2. Another version uses a fused salt technique to deposit the platinum layer followed by a high activity-low temperature aluminizing treatment. This latter coating includes a thick Pt.sub.2 Al.sub.3 plus PtAl structured zone.
Platinum-enriched diffused aluminide coatings have been tested on nickel and cobalt base superalloys and have been found to exhibit better hot corrosion/ oxidation resistance than the unmodified, simple diffused aluminide coatings on the same substrates. However, the platinum-enriched diffused aluminide coatings have exhibited reduction in coating ductility and undesirable increase in ductile-to-brittle transition temperature (DBTT) as compared to the unmodified, simple diffused aluminide coatings.
It has been proposed to improve the hot corrosion/oxidation resistance of diffused aluminide coatings by alloying the coating with silicon. In particular, the application of a high purity silicon slurry spray followed by a pack aluminizing treatment has been reported to improve the hot corrosion/ oxidation resistance of nickel-base superalloys. However, the addition of silicon to the diffused aluminide coating has also been reported to reduce the ductility of the coating.
It is an object of the present invention to provide a method for applying a hot corrosion- and oxidation-resistant platinum-silicon-enriched diffused aluminide coating to nickel and cobalt base superalloy substrates in such a manner as to reduce the overall cost of coating application. It is another object of the present invention to increase the ductility of a platinum-enriched diffused aluminide coating at elevated temperatures without compromising hot corrosion and oxidation resistance by the inclusion of both platinum and silicon in the coating.